There is a wireless communication technique that improves the communication quality by using an antenna including a plurality of antenna elements for a transmitting and receiving antenna mounted on a communication apparatus such as a mobile information terminal apparatus. Multiple-input Multiple-output (MIMO) is an example of such a wireless technique.
An antenna including a plurality of antenna elements is designed by an antenna design method such as the one below, for example.
First, the designer creates a model of the antenna including a plurality of antenna elements alone.
The designer obtains the antenna characteristic of the created antenna-alone model by a simulation using an electromagnetic field simulator. The antenna characteristic to be obtained includes the antenna impedance, radiation efficiency, and total efficiency, and the like.
Next, the designer creates the models of matching circuits which are to be connected to the plurality of the antenna elements, respectively, with reference to the obtained characteristic of the antenna-alone model. Then, the designer adds the respective created models of matching circuits to the antenna-alone model, and obtains the antenna characteristic of the model to which the matching circuits have been added, using the electromagnetic field simulator.
The designer evaluates whether or not the obtained antenna characteristic satisfies a desired standard. When the antenna characteristic of the antenna model to which the matching circuits have been added satisfies the desired standard according to the evaluation, the designer finishes the antenna design. On the other hand, when the antenna characteristic does not satisfy the desired standard according to the evaluation, the designer creates the respective models of matching circuits to be connected to the plurality of antenna elements again, and continues the antenna design.
Meanwhile, there is a conventional technique to measure the radiation characteristic of a patch antennal using a prescribed parameter including the electric conductivity of the transparent conductive film to be the radiation element and the electric conductivity of the microstrip line.
There is a conventional technique to measure the radiation efficiency of the matching element and the antenna of the transmitting and/or receiving module.
There is a conventional technique to obtain the relationship between the frequency and the return loss and the relationship between the frequency and the antenna efficiency about a notch antenna including a reactance circuit by an electromagnetic field simulation such as the Finite Difference Time Domain Method (FDTD).
In recent years, mobile information terminal apparatus has been becoming smaller and thinner, and the mounting space of the antenna provided in the mobile information terminal apparatus has been becoming smaller and thinner as well. In the smaller and thinner mobile information terminal apparatus, the antenna and peripheral parts composed of metal may be close in the structure. When there is metal near the antenna, an electric current that cancels out the antenna current flows into the metal, which lead to the deterioration of the antenna performance.
The performance index includes radiation resistance Rr. Assuming the loss resistance included in the antenna and the matching circuit and the like as R1, radiation efficiency η which is the ratio of the net electric power added to the antenna to the electric power radiated from the antenna is expressed by expression (1) below.
                    η        =                              R            r                                              R              r                        +                          R              l                                                          (        1        )            
As is apparent from expression (1), when radiation resistance Rr of the antenna is small, radiation efficiency η deteriorates to a large extent even if loss resistance R1 is a little value. Therefore, it is desirable to design the antenna so that radiation resistance Rr becomes large.
However, in the smaller and thinner mobile information terminal apparatus, metal may be present near the antenna as described above, and it may become a structure which is difficult to radiate electric wave, that is, a structure in which radiation resistance Rr is small. Therefore, in designing an antenna in the structure in which radiation resistance Rr is small, there is a need to pay attention to loss resistance R1 even if it is a small value.
When the input impedance of the antenna deviates from the characteristic impedance (for example, 50Ω), it is needed to match by a matching circuit. Matching elements that constitute the matching circuit include a capacitance component or an inductance component, and a parasitic inductance component or a parasitic capacitance component, as well as a resistance component though it is a little. Therefore, in designing an antenna having a structure in which radiation resistance Rr is small, there is a need to pay attention to the resistance component of each matching element that constitute the matching circuit.
In addition, when the antenna including a plurality of antennas as described above is mounted on the smaller and thinner mobile information terminal apparatus, the allocated gap between antenna elements becomes small, and electromagnetic coupling may be generated between the antenna elements. When electromagnetic coupling is generated between the antenna elements, it leads to the deterioration of the antenna characteristic such as a decrease in the radiation efficiency. Therefore, when the antenna for a design target includes a plurality of antenna elements, there is a need to design the antenna in consideration of an interaction generated between the antenna elements such as the isolation characteristic between the antenna elements.
However, by the conventional antenna design method described above, it is impossible to design the antenna in a speedy and efficient manner in view of the interaction between the antenna elements and the loss in the matching circuit connected to the individual antenna element.
That is, in the conventional antenna design method, the antenna characteristic of the antenna model including the matching circuits which are connected to the plurality of antenna elements, respectively, are obtained by a simulation. For this reason, in the conventional antenna design method, it requires a huge amount of time to calculate the antenna characteristic in view of the interaction between the antenna elements and the loss in the matching circuit. Therefore, by the conventional antenna design method, it is impossible to design the antenna that satisfies the desired standard, in a speedy manner.
In addition, in the conventional antenna design method described above, the designer evaluates whether or not the simulation result matches the desired standard, and the designer needs to reconsider the configuration of the matching circuit based on the evaluation result. For this reason, it is impossible to design the antenna in a speedy and efficient manner in view of the interaction between the antenna elements and the loss in the matching circuit.